Preservative material and storage method for liquid

ABSTRACT

A nanofiber structure having pores is used as a preservative material for a liquid material. A liquid material can be stored for a long period without causing deterioration and without the need of using any additive such as an anti-bacterial agent merely by contacting the liquid material with the preservative material.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 12/989,880, filed on Oct. 27, 2010, which is a 371 of PCT/JP2009/061896, filed Jun. 30, 2009.

TECHNICAL FIELD

The present invention relates to a preservative material and storage method for liquids. More specifically, the invention relates to a preservative material for liquids which is composed of a nanofiber material, and to a method of storing liquids using the same.

BACKGROUND ART

Liquids that have been placed in containers, such as eye drops, cosmetics, toiletries, beverages and inks, are sometimes used over a relatively long period of time after the container is opened.

In such cases, to prevent the quality of the liquid from deteriorating during the period of use due to airborne microorganisms and falling microorganisms and fungal spores which enter the container and grow therein, in addition to the ingredients for achieving the intended effects of the contents themselves, use is also generally made of additives such as antibacterial agents and preservatives.

Synthetic compounds such as synthetic preservatives have often been used as such additives, but because of the rise in the safety consciousness of the consumer in recent years, the preference nowadays is for the use of products of natural origin (see Patent Documents 1 to 4).

However, additives such as preservatives and antiseptic agents not only lead to a decline in the objects and effects of the contents themselves, various other problems arise, such as an undesirable odor or taste and color, adverse effects on the human body (e.g., chapped skin, discomfort, skin irritation), the extra cost of the additives, and an increase in the elements of quality control.

For example, when a synthetic compound is added, depending on the physical constitution of the user, this may give rise to a hypersensitivity reaction. When this happens, even if the chief ingredients are harmless in humans, people who develop a hypersensitivity reaction are unable to use the liquid.

As for products of natural origin, these in themselves sometimes have a distinctive odor or taste and color, which often limits the products in which they can be used. Moreover, there are not many types of naturally occurring products which can actually be used.

PRIOR-ART DOCUMENTS Patent Documents

Patent Document 1: JP-A 2001-178431

Patent Document 2: JP-A 2000-229804

Patent Document 3: JP-A 6-70730

Patent Document 4: JP-A 2004-59525

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention was arrived at in light of the above circumstances. The objects of the invention are to provide a preservative material for liquids which is capable of preserving a liquid for a relatively long period of time without the addition of additives such as antibacterial agents, and to provide a method of storing liquids using such a material.

Means for Solving the Problems

The inventors have conducted extensive investigations in order to achieve the above objects. As a result, they have discovered that by bringing a nanofiber material having numerous pores in contact with a liquid, the growth of microorganisms within the liquid is suppressed or microorganisms within the liquid are destroyed. Moreover, they have found that this nanofiber material can be advantageously used as a preservative material for liquids.

Accordingly, the present invention provides:

-   1. A preservative material for liquids, comprising a nanofiber     material having a plurality of pores. -   2. A storage method for liquids, comprising the step of contacting     the preservative material for liquids of 1 above with a liquid. -   3. A storage method for liquids, comprising the step of holding, in     a nanofiber material having a plurality of pores, a liquid in an     amount not greater than a void volume of the nanofiber material. -   4. A storage container for liquids, comprising at least one opening     and an interior in which a liquid is placed, wherein a nanofiber     material is disposed at the interior in such a way as to contact the     liquid. -   5. A storage container for liquids, comprising at least one opening     and an interior in which a liquid is placed, wherein a filter having     at least one layer of a nanofiber material is provided at the     opening in such a way as to isolate the interior of the container     from the exterior. -   6. A liquid-containing nanofiber material comprising a nanofiber     material having a plurality of pores and a liquid which is held in     at least some portion of the pore voids.

Effects of the Invention

This invention enables liquids, such as pharmaceuticals, cosmetics, toiletries, oral hygiene agents, beverages, items of stationery, liquid cultures and liquid manure, to be stored for an extended period of time without the addition of additives such as antibacterial agents. That is, the invention is able, without the use of additives, to suppress microbial toxins, odors and the like which are generated by the growth of microorganisms.

The preservative material for liquids of the invention renders unnecessary additives such as antibacterial agents that have hitherto been used, thereby making it possible not only to eliminate deterioration in the inherent effects of liquids such as pharmaceutical products and in the odor, taste, color and the like of such liquids, but also to store these liquids in a condition that is safe for the human body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is described more fully below.

The preservative material for liquids according to the invention is composed of a nanofiber material having a plurality of pores.

Here, the shape of nanofiber material is not subject to any particular limitation, provided it has numerous pores. Examples include cotton wool-like, nonwoven fabric-like, felt-like and sponge-like nanofiber materials. In this case, blending or covering with fibers having a fiber diameter of 1 μm or more may be carried out by a known technique.

The nanofibers account for a proportion (weight ratio) of the nanofiber material which is not subject to any particular limitation but, in order to fully elicit the effects of the invention, is preferably more than 50 wt %, more preferably at least 70 wt %, and even more preferably at least 80 wt %.

The fibers making up the nanofiber material have an average fiber diameter of at least 1 nm but less than 1,000 nm, preferably from 10 to 800 nm, and more preferably from 50 to 700 nm.

The basis weight of the nanofiber material, although not subject to any particular limitation, is preferably from about 1 to about 100 g/m², and especially from about 10 to about 70 g/m².

In addition, the diameter of the pores in the nanofiber material, although not subject to any particular limitation, may be set to, for example, from about 0.001 to about 100 μm, preferably from about 0.01 to about 10 μm, and more preferably from about 0.01 to about 5 μm.

When the above nanofiber material is used as the subsequently described filter, by setting the minimum pore size therein to 0.1 μm or less, preferably from 0.01 to 0.1 μm, and more preferably from 0.01 to 0.08 μm, and by setting the maximum pore size to more than 0.1 μm but not more than 1 μm, preferably more than 0.2 μm but not more than 1 μm, and more preferably from 0.3 to 1 μm, microorganisms and the like present outside of the container can be prevented from entering the interior of the container.

The starting polymer for the nanofibers is not subject to any particular limitation, provided it is a water-insoluble polymer. Illustrative examples include polyester resins, polyamide resins, polyurethane resins, polyacrylic resins, polyamideimide resins, polyvinyl chloride resins, polystyrene resins, polyimide, polyarylate, polyaniline, polypyrrole, polythiophene, cellulose and cellulose derivatives.

The nanofibers used in the invention may be obtained by spinning a solution (composition) of the above polymer dissolved in a suitable solvent using any of various spinning processes, such as electrostatic spinning, spunbonding, melt blowing and flash spinning.

In the practice of the invention, the use of an electrostatic spinning process, which is capable of manufacturing the fibers to a relatively uniform diameter in a range of at least 1 nm but less than 1,000 nm, is especially preferred.

Electrostatic spinning is a process in which, as an electrically charged electrostatic spinning dope (resin solution) is spun within an electrical field, the dope is broken up by forces of repulsion between the electric charges, resulting in the formation of a very fine fibrous material composed of the resin.

The basic configuration of the apparatus which carries out electrostatic spinning includes a first electrode which also serves as a nozzle for discharging the dope to be electrostatically spun and which applies to the dope a high voltage of from several thousands to several tens of thousands of volts, and a second electrode which faces the first electrode. The dope which has been ejected or shaken from the first electrode becomes nanofibers due to the high-speed jets and the subsequent folding and expansion of the jets within the electrical field between the two opposed electrodes, and collects on the surface of the second electrode, thereby giving nanofibers (nanofiber material).

The solvent used in preparing the dope for electrostatic spinning is not subject to any particular limitation, provided it is able to dissolve the polymer. Illustrative examples of suitable solvents include acetone, methanol, ethanol, propanol, isopropanol, toluene, benzene, cyclohexane, cyclohexanone, tetrahydrofuran, dimethylsulfoxide, 1,4-dioxane, carbon tetrachloride, methylene chloride, chloroform, pyridine, trichloroethane, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, ethylene carbonate, diethyl carbonate, propylene carbonate, acetonitrile, and organic acids such as formic acid, lactic acid and acetic acid. These solvents may be used singly or as mixtures of two or more thereof.

The storage method for liquids according to the invention involves bringing the preservative material for liquids described above into contact with a liquid.

Here, the preservative material and the liquid may come into contact in any suitable manner; the liquid may come into contact with only part of the surface of the preservative material, or (a portion of) the liquid may be impregnated into and held by at least some portion of the voids of the plurality of pores in the preservative material.

The amount of preservative material used is not subject to any particular limitation, provided it is an amount that ensures a sufficient probability of contact with microorganisms. Specifically, it is preferable to use at least 1 mg, more preferably at least 5 mg, and even more preferably at least 10 mg, of the preservative material per 500 mL of liquid.

Moreover, in an embodiment wherein a liquid is held in at least some portion of the voids of the plurality of pores in the nanofiber material making up the preservative material, the liquid may be held in an amount not greater than the void volume of the nanofiber material so as to give a liquid-containing nanofiber material.

In such a liquid-containing nanofiber material, because deterioration of the liquid held at the interior of the voids does not readily arise, the nanofiber material can be advantageously employed as, for example, sanitizing wipes, wet tissues, make-up puffs and sponges which are used over a relatively long period of time after being opened.

The liquid which is used for storage is exemplified in particular by liquids composed primarily of water, including pharmaceutical products such as eye drops, medicinal drinks and sprays; cosmetics such as toners, lotions, tonics, shampoos and rinses; liquids used in toiletries; liquids used in stationery, such as India ink and other types of ink; liquid cultures, liquid manure and water placed in vases; and liquids for drinking, such as drinking water, long shelf life water, juices and alcoholic beverages.

The storage container for liquids according to the present invention is an ordinary liquid storage container having at least one opening and an interior in which a liquid is placed, wherein a nanofiber material is disposed at the interior in such a way as to contact the liquid.

Here, the place where the nanofiber material is disposed is not subject to any particular limitation, provided it is a position that enables the nanofiber material to come into contact with the liquid. Examples include any place within the container, such as the inside wall or the bottom surface. However, because the liquid placed at the interior decreases with use, to enable constant contact between the liquid and the nanofiber material, when the material is fixed to the container, it is preferable to dispose the material in a manner so as to include the bottom surface of the container. Alternatively, the nanofiber material may be simply immersed or allowed to float in the liquid without being fixed to the container.

Specific modes of use are exemplified by those in which the nanofiber material is attached to the inside wall and bottom surface of a container such as a PET bottle, glass bottle, vase or eye drop container, or in which the nanofiber material is simply dropped into these containers.

Also, the inventive storage container for liquids may be one in which a filter having at least one layer of a nanofiber material is provided at the opening of the above ordinary liquid storage container in such a way as to separate the interior of the container from the exterior.

Here, the liquid and the nanofiber material come into contact when the liquid flows into the interior through the opening and/or when the liquid flows out to the exterior, thereby eliciting the antibacterial or disinfecting effects of the invention. As a result, liquid preserving effects are manifested.

When the above nanofiber material is used as a filter, as explained above, by adjusting the minimum pore diameter and the maximum pore diameter within specific ranges, the entry of microorganisms from the exterior can be blocked. Therefore, through the synergism of the sterilizing effects on microorganisms in the liquid and the blocking effect against organisms from the exterior, it is possible to preserve liquids for a longer period of time.

Moreover, it is possible to dispose a nanofiber material at the interior of the container, and also provide a filter which includes a nanofiber material.

The nanofiber material may be used alone as a filter, or it may be laminated with another nonwoven fabric or porous film or sheet and the resulting laminate used as a filter.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation. Tests and measurements in the Examples and Comparative Examples below were carried out by the following methods.

[1] Average Fiber Diameter

The fiber diameter was measured at 20 places selected at random from a micrograph obtained by capturing an image of the specimen surface at a magnification of 5000× with a scanning electron microscope (S-4800I, manufactured by Hitachi High-Technologies Corporation). The average (n=20) for all the fiber diameters was calculated and treated as the average fiber diameter.

[2] Antibacterial Activity Measuring Test 1 (Cell Count Measurement Method)

The test was carried out using the following cell count measurement method (JIS L 1902) described in K•kin B•sh•Kak•Seihin no Kak•K•ka Hy•ka Shiken Manyuaru [Manual for Evaluating and Testing the Effects of Treatment in Antibacterial Deodorization-Finished Products] established by the Sen'i Seihin Eisei Kak•Ky•gikai (Japanese Association for the Hygienic Finishing of Textiles).

A suspension of Staphylococcus aureus as the test organism was initially prepared by culturing this organism in a common bouillon medium and adjusting the concentration to from 10⁶ to 10⁷ cells/mL. The suspension (0.2 mL) was uniformly inoculated onto 0.4 g of the specimen in a sterilized threaded vial and static cultured at 36 to 38° C. for 18 hours, following which 20 mL of sterile, buffered physiological saline was added to the vessel and the vessel contents were shaken vigorously by hand 25 to 30 times at an amplitude of 30 cm so as to disperse the live cells in the specimen within the liquid. Next, a suitable dilution series was created with sterile, buffered physiological saline, 1 mL of dilution at each stage was placed in two Petri dishes, and about 15 mL of standard agar culture medium was added. Culturing was then carried out at 36 to 38° C. for 24 to 48 hours, following which the number of live colonies was counted and the live cell count in the specimen was computed in accordance with the degree of dilution. In rating the effects, the test was judged to be complete when the growth value exceeded 1.5. The bacteriostatic activity S and the bactericidal activity L were determined from the following formulas.

Bacteriostatic activity S=B−C

Bactericidal activity L=A−C

where A: average common log value of live cell count for three specimens immediately after contacting a standard cloth with test organisms

-   -   B: average common log value of live cell count for three         specimens after culturing standard cloth for 18 hours     -   C: average common log value of live cell count for three         specimens after culturing antibacterially finished specimen for         18 hours

[3] Antibacterial Activity Measuring Test 2 (Antibacterial Finished Product—Antibacterial Test Method; JIS Z 2801)

S. aureus, Escherichia coli and Klebsiella pneumoniae as the test organisms were initially cultured on a common agar medium (Nissui Pharmaceutical Co., Ltd.), and the grown colonies were scraped off and suspended in a 1/500 concentration common bouillon medium (Eiken Chemical Co., Ltd.) to give test organism suspensions adjusted to about 10⁶ CFU (colony forming units)/mL.

A 50 mm square reinforced polyethylene film was placed in a sterile Petri dish, and the specimen (40 mm square, 0.1 g) was placed on top thereof. The test organism suspension (0.1 mL) was added dropwise to the specimen, and a 50 mm square reinforced polyethylene film was covered over and brought into close contact with the specimen. This state is one in which an amount of liquid not greater than the void volume of the specimen is held in the specimen. The inoculated specimen was then placed in a closed vessel held at a relative humidity of at least 90% RH and a temperature of 35±1° C., and allowed to act for 24 hours. After 24 hours of action, the specimen was recovered in a sterile stomacher bag, 10 mL of a soybean casein digest broth with lecithin and polysorbate (SCDLP) bouillon medium (Eiken Chemical Co., Ltd.) was added, and the test organisms were washed out. Using the washing as the stock solution, a ten-fold dilution series was prepared. One milliliter each of this sample stock solution and of the dilutions were prepared as pour plates with a standard agar medium (Nissui Pharmaceutical Co., Ltd.) and cultured at 35±1° C. for 48 hours, following which the colonies that grew on the medium were counted and the number of test organisms for each specimen was determined.

[4] Antibacterial Activity Measuring Test 3 (Antibacterial Activity Evaluating Test for Airborne Organisms)

Using a 1 m³ test chamber made of polyvinyl chloride, two stirring fans were placed in diagonally opposed corners on the floor surface. A bacterial suspension spraying hole and an airborne bacteria collecting hole were provided at the center of one sidewall of the test chamber, a bacterial suspension spraying device and a filter holder were connected thereto, and an airborne bacteria collecting unit was connected after the filter holder. A glass nebulizer containing a test organism suspension was used as the bacterial suspension spraying device, and a glass midget impinger was used as the airborne bacteria collecting unit.

S. aureus as the test organism was cultured in tryptic soy agar (TSA medium; available from Difco). The grown colonies were scraped off and suspended in sterile ion-exchanged water to form a test organism suspension adjusted to about 10⁹ cells/mL.

Compressed air was delivered to the bacterial suspension-containing glass nebulizer from a compressor and the test organism suspension was sprayed into the test chamber at a rate of 0.2 mL/min for 10 minutes, thereby suspending the bacteria in air (the airborne cell count was about 2×10⁹ cell/1,000 L). The suspended bacteria within the chamber were collected through a specimen (diameter, 49 mm) placed in the filter holder. That is, a glass midget impinger in which 20 mL of sterile physiological saline had been placed was connected after the filter holder, and air within the test chamber was collected each time at a rate of 5 L/min for 10 minutes (=50 L). As a control, the airborne bacteria within the chamber were collected through a filter holder in which a specimen had not been placed.

Using as the stock solution the sterile physiological saline within the impinger after the test organisms had been collected, a ten-fold dilution series was prepared. One milliliter each of this sample stock solution and of the dilutions were prepared as pour plates with a TSA medium and cultured at 35° C. for 48 hours, following which the grown colonies were counted and the number of organisms that passed through the specimen per 50 L of air was determined.

At the same time that the attached cells were passed through the specimen in this way, the cells that attached to the specimen were subjected to the subsequently described evaluation as specimen-attached cells. Air within the test chamber containing suspended cells was passed through the specimen placed in the filter holder at a rate of 5 L/min for 10 minutes (=50 L), causing the test organisms to attach to the specimen. The specimen to which test organisms had been made to attach was removed from the filter holder, placed in a closed vessel filled with the vapor of a 1.8 wt % sodium chloride solution (35° C.; relative humidity, 90%), and held for 24 hours.

The test specimens were then removed, each was placed in a sterile stomacher bag, 10 mL of SCDLP medium (Eiken Chemical Co., Ltd.) was added, and the attached cells were washed out. Using the washing as the stock solution, a ten-fold dilution series was created. One milliliter each of this sample stock solution and of the dilutions were prepared as pour plates with TSA medium and cultured at 35° C. for 48 hours, following which the number of colonies that grew on the medium were counted and the number of attached cells was determined.

[5] Antibacterial Efficacy Measuring Test

Specimens were placed in amounts of 10 mg, 0.1 g or 0.2 g in a sterilized wide-mouth bottle (mouth diameter, 3 cm), and 500 mL of pre-sterilized drinking water was placed in each bottle so that the drinking water was in contact with all portions of the specimen. These bottles were left to stand uncapped at room temperature for 30 minutes, following which they were capped and held at room temperature for two weeks. After two weeks, 200 μL of each solution was inoculated onto a standard agar medium, incubated at 37° C. for 48 hours, and the colonies that formed were observed. The antibacterial properties were evaluated based on the common bacteria within the solution.

−: no change (no colonies formed)

+: changed (colonies formed)

[6] Skin Toner Storage Efficacy Test

S. aureus and E. coli as the test organisms were initially cultured in a common agar medium (Nissui Pharmaceutical Co., Ltd.), and the grown colonies were scraped off and suspended in a 1/500 concentration common bouillon medium (Eiken Chemical Co., Ltd.) to give test organism suspensions adjusted to about 10⁷ CFU (colony forming units)/mL. The test organism suspension (0.1 mL) was added to 100 mL of skin toner (FDR Lotion M, available from Fancl Corporation), and a toner test solution for each organism was prepared to a cell concentration of 10⁴ CFU/mL. The toner test solutions were each dispensed in an amount of 30 mL to a 50 mL centrifuge tube and a given amount of the specimen was placed therein, following which the tubes were stored at room temperature for a given number of days (0, 1, 3 or 7 days). The cell counts for the toner test solutions were measured by the pour plate method after the respective number of days of storage had elapsed. As a control, a similar test was carried out on a skin toner test solution in which the specimen was not placed.

[7] Minimum Pore Size and Maximum Pore Size Measurement Test

The pore sizes were measured and evaluated as described below based on the bubble point method (ASTM F316, JIS K 3832).

Using a perm porometer (model CFP-1200A manufactured by PMI), dry air was passed through a sample having a measurement diameter of 25 mm and the air flow rate was observed as the air pressure was increased in stages (dry flow rate curve).

Next, the sample was soaked in Galwick (available from PMI) having a surface tension of 16 dynes/cm, and the soaked sample was pretreated by degassing in a vacuum drier so that no bubbles remained in the sample. Dry air was passed through the pretreated sample, and the air flow rate was observed as the air pressure was increased in stages (wet flow rate curve).

The minimum pore size and the maximum pore size were determined from these two dry and wet flow rate curves.

[1] Production of Preservative Material for Liquids Example 1 Polylactic Acid

Ten parts by weight of polylactic acid resin (LACEA H280, available from Mitsui Chemicals, Inc.) and 45 parts by weight of dimethylformamide (abbreviated below as “DMF”) were mixed and heated to 60° C., thereby dissolving the polylactic acid resin in the DMF and obtaining 55 parts by weight of a polylactic acid-containing solution (solids content, 18 wt %).

This lactic acid-containing solution (spinning dope) was placed in a syringe and electrostatic spinning was carried out at a discharge tip orifice diameter of 0.4 mm, an applied voltage of 20 KV (at room temperature and atmospheric pressure), and a distance from the discharge tip orifice to the fibrous substance collecting electrode of 15 cm, thereby giving a preservative material for liquids (nanofiber nonwoven fabric).

The resulting nonwoven fabric had an average fiber diameter of 500 nm, and fibers with a diameter greater than 3 μm were not observed.

Example 2 Nylon 6

Ten parts by weight of nylon 6 (A1030BRT, produced by Unitika, Ltd.) was dissolved in 57 parts by weight of formic acid at room temperature (25° C.), thereby obtaining 67 parts by weight of a nylon 6-containing solution (solids content, 15 wt %).

This nylon 6-containing solution (spinning dope) was placed in a syringe and electrostatic spinning was carried out at a discharge tip orifice diameter of 0.4 mm and an applied voltage of 50 KV (at room temperature and atmospheric pressure), thereby giving a preservative material for liquids (nanofiber nonwoven fabric). The resulting nonwoven fabric had an average fiber diameter of 250 nm, and fibers with a diameter greater than 1 μm were not observed.

Example 3 Polyacrylonitrile

Ten parts by weight of polyacrylonitrile (Barex 1000S; available from Mitsui Chemicals, Inc.) was dissolved in 40 parts by weight of DMF at room temperature (25° C.) to give 50 parts by weight of a polyacrylonitrile-containing solution (solids content, 20 wt %).

This polyacrylonitrile-containing solution (spinning dope) was placed in a syringe and electrostatic spinning was carried out at a discharge tip orifice diameter of 0.4 mm, an applied voltage of 30 KV (at room temperature and atmospheric pressure), and a distance from the discharge tip orifice to the fibrous substance collecting electrode of 15 cm, thereby giving a preservative material for liquids (nanofiber nonwoven fabric).

The resulting nonwoven fabric had an average fiber diameter of 100 nm, and fibers with a diameter greater than 1 μm were not observed.

Example 4 Cellulose

A cuprammonium solution was prepared by weighing out 0.768 g of copper hydroxide (Wako Pure Chemical Industries) into a flask, then adding 17.86 g of a 28% aqueous ammonia solution (Wako Pure Chemical Industries) and 1.372 g of water. One part by weight of absorbent cotton (Hakujuji Co., Ltd.) was added to 20 parts by weight of this solution, thereby giving 21 parts by weight of a cellulose-containing solution (solids content, about 4.8 wt %). The solution was stirred for 18 hours at room temperature, and the starting cotton was confirmed to have completely dissolved.

This cellulose-containing solution (spinning dope) was placed in a syringe and electrostatic spinning was carried out at a discharge tip orifice diameter of 0.4 mm, an applied voltage of 30 KV (at room temperature and atmospheric pressure), and a distance from the discharge tip orifice to the fibrous substance collecting electrode of 10 cm, thereby giving a nanofiber nonwoven fabric. The resulting nanofiber nonwoven fabric was washed with 0.1 mol/L hydrochloric acid so as to remove the copper ions, thereby giving the target preservative material for liquids (nanofiber nonwoven fabric).

The resulting nonwoven fabric had an average fiber diameter of 700 nm, and fibers with a diameter greater than 1.5 μm were not observed.

Comparative Example 1 Polylactic Acid

The same polylactic acid resin as in Example 1 was melt spun at a spinning temperature of 160° C. using monofilament nozzles, thereby giving filaments having an average diameter of 20 μm. The resulting filaments were separated and dispersed, then deposited on a moving conveyor screen-type condenser to form a web. Next, the constituent filaments were united with each other by subjecting the web to a conventional nonwoven fabric-forming operation, thereby giving a nonwoven fabric. The average fiber diameter was 20,000 nm.

Comparative Example 2 Nylon 6

The same nylon 6 resin as in Example 2 was melted at a spinning temperature of 260° C., and formed into a nonwoven fabric by the same process as in Comparative Example 1. The average fiber diameter was 15,000 nm.

Comparative Example 3 Polyacrylonitrile

The same polyacrylonitrile resin as in Example 3 was melted at a spinning temperature of 260° C. using a pressure melter-type melt spinning machine, and formed into a nonwoven fabric by the same process as in Comparative Example 1. The average fiber diameter was 30,000 nm.

Comparative Example 4 Cellulose

The same absorbent cotton (Hakujuji Co., Ltd.) as in Example 4 was used. A web was formed by a conventional wet method, and a nonwoven fabric was obtained by directing a high-pressure stream of water at the cotton to mutually entangle the fibers. The average fiber diameter was 30,000 nm.

Antibacterial Activity Measuring Tests 1 to 3, the antibacterial efficacy measuring test, the skin toner storage efficacy test (Examples 2 and 3 only), and the minimum pore size and maximum pore size measurement test were carried out on the nonwoven fabrics obtained in Examples 1 to 4 and Comparative Examples 1 to 4. The results of Antibacterial Activity Measuring Test 1 are shown in Table 1, the results of Antibacterial Activity Measuring Test 2 are shown in Table 2, the results of Antibacterial Activity Measuring Test 3 are shown in Table 3 (passed cell count) and Table 4 (antibacterial activity against attached cells), the results of the antibacterial efficacy measuring test are shown in Table 5, the results of the skin toner storage efficacy test are shown in Table 6 (S. aureus) and Table 7 (E. coli), and results of the minimum pore size and maximum pore size measurement test are shown in Table 8.

TABLE 1 Average fiber Antibacterial tests* diameter Live cell Bacteriostatic Bactericidal (nm) Resin count activity activity Example 1 500 polylactic acid <600 >4.1 >1.5 2 250 nylon 6 <600 >4.1 >1.5 3 100 polyacrylonitrile <600 >4.1 >1.5 4 700 cellulose <600 >4.1 >1.5 Comparative 1 20,000 polylactic acid 4.5 × 10⁷ −0.3 −2.9 Example 2 15,000 nylon 6 7.4 × 10⁷ −0.3 −2.5 3 30,000 polyacrylonitrile 7.2 × 10⁷ −0.3 −1.8 4 30,000 cellulose 7.9 × 10⁷ −0.3 −2.5 *Standard cloth (cotton) Immediately after inoculation: 1.9 × 10⁴ After 18 hours of culturing: 7.9 × 10⁶

TABLE 2 Average fiber Live cell count diameter Staphylococcus Klebsiella Escherichia (nm) Resin aureus pneumoniae coli Example 1 500 polylactic acid <10 <10 — 2 250 nylon 6 <10 <10 48 3 100 polyacrylonitrile <10 <10 1,800 4 700 cellulose <10 <10 <10 Comparative 1 20,000 polylactic acid 380,000 1,500,000 — Example 2 15,000 nylon 6 400,000 1,400,000 — 3 30,000 polyacrylonitrile 400,000 1,400,000 — 4 30,000 cellulose 420,000 1,600,000 20,000,000

TABLE 3 Passed cell count Average Passed cell count fiber diameter (nm) Resin (cells/50 L of air) Example 1 500 polylactic acid <10 2 250 nylon 6 <10 3 100 polyacrylonitrile <10 4 700 cellulose <10 Comparative 1 20,000 polylactic acid 1,500,000 Example 2 15,000 nylon 6 1,200,000 3 30,000 polyacrylonitrile 1,400,000 4 30,000 cellulose 1,600,000

TABLE 4 Antibacterial activity against attached cells Average fiber diameter (nm) Resin Attached cell count Example 1 500 polylactic acid <10 2 250 nylon 6 <10 3 100 polyacrylonitrile <10 4 700 cellulose <10 Comparative 1 20,000 polylactic acid 3,300,000 Example 2 15,000 nylon 6 2,100,000 3 30,000 polyacrylonitrile 4,200,000 4 30,000 cellulose 4,200,000

TABLE 5 Average fiber Amount of diameter preservative material (nm) Resin 10 mg 0.1 g 0.2 g Example 1 500 polylactic acid − − − 2 250 nylon 6 − − − 3 100 polyacrylonitrile − − − 4 700 cellulose − − − Comparative 1 20,000 polylactic acid + + + Example 2 15,000 nylon 6 + + + 3 30,000 polyacrylonitrile + + + 4 30,000 cellulose + + + Control: distilled water only +

TABLE 6 Skin toner storage efficacy test (S. aureus) Specimen S. aureus count Weight (g)/ After After After 30 mL of 1 day 3 days 7 days test of of of Resin solution 0 days storage storage storage Example 2 nylon 6 0.4 — 16,000 2,000 <10 2 nylon 6 0.2 — 19,000 4,000 <10 3 polyacrylonitrile 0.4 — 12,000 <10 <10 3 polyacrylonitrile 0.2 — 12,000 260 <10 Control — — 37,000 22,000 14,000 8,400 Control: Skin toner test solution only

TABLE 7 Skin toner storage efficacy test (E. coli) Specimen E. coli count Weight (g)/ After After After 30 mL of 1 day 3 days 7 days test of of of Resin solution 0 days storage storage storage Example 2 nylon 6 0.4 — 21,000 7,000 <10 2 nylon 6 0.2 — 26,000 7,600 40 3 polyacrylonitrile 0.4 — 20,000 230 <10 3 polyacrylonitrile 0.2 — 23,000 2,400 <10 Control — — 44,000 22,000 22,000 11,000 Control: Skin toner test solution only

TABLE 8 Average fiber diameter Pore size (μm) (nm) Resin Minimum Maximum Example 1 500 polylactic acid 0.0432 0.7070 2 250 nylon 6 0.0176 0.501 3 100 polyacrylonitrile 0.0210 0.8560 4 700 cellulose 0.0500 0.9500 Comparative 1 20,000 polylactic acid 15.0 102.0 Example 2 15,000 nylon 6 9.0 80.5 3 30,000 polyacrylonitrile 19.0 132.3 4 30,000 cellulose 19.0 132.3

It is apparent from Tables 1 to 4 that the preservative materials obtained in Examples 1 to 4 had antibacterial activities, and it is apparent from Tables 5 to 7 that bacteria did not grow in solutions stored using these preservative materials. Moreover, it is apparent from Table 3 that the preservative materials used in Examples 1 to 4 allowed substantially no bacteria to pass through. 

1. A storage method for liquids, comprising the step of contacting a preservative material for liquids with a liquid, said preservative material comprising a nanofiber material having a plurality of pores.
 2. The method of claim 1, wherein the liquid is held in the nanofiber material in an amount not greater than a void volume of the nanofiber material.
 3. The method of claim 1 or 2, wherein the shape of the nanofiber material is cotton wool-like, nonwoven fabric-like, felt-like or sponge-like.
 4. A storage container for liquids, comprising at least one opening and an interior in which a liquid is placed, wherein a nanofiber material having a plurality of pores is disposed at the interior in such a way as to contact the liquid.
 5. The storage container of claim 4, wherein the nanofiber material is disposed on an inside wall or a bottom surface thereof. 